Depends on the circumstance.
For example, do we have to physically kill one or the other? The act of killing and the act of letting a being die are morally separate.
Are both drowning and we can only save one? The human is the choice. One argument is that humans are inherently more valuable.
But let's say you don't think that. Let's say humans and animals are equal- both have the same right to life. You still need to pick the human. An animal dying in this viewpoint causes the same amount of inherent pain or dissatisfaction as a human dying. However, humans have preferences outside of simply eating, mating, and living. By letting a person die instead of an animal, you are violating more of these preferences than if you let the animal die. Thus whether or not you believe the animal is worth less, the human is the clear choice.
If you'd like to take a Utilitarian viewpoint instead, you could say that an animal dying and human dying cause the same amount of pain themselves. However, a human dying deprives the person of a much greater number of pleasures than the animal would have lost, and other living beings may very well be affected by this death on a much deeper emotional level than, say, two chipmunks. Thus... you still have to pick the human because picking the human to live ends up with the greatest amount of good and the least amount of pain.
You could make it more interesting by saying 'a human or an equally advanced alien' or something.
[Edited on 12.01.2012 11:04 PM PST]